An internal gear oil pump is so configured as to be driven by rotating a shaft fixed to the center of an inner rotor. Then, the inner rotor is securely fixed to the shaft in order to avoid idling being caused by a driving torque of the pump.
Meanwhile, keyway, caulking, press-fitting or the like is being used in general for fixation between an inner rotor and a shaft. However, fixing by way of keyway or caulking has had a problem that a production cost increases due to the increase of assembly processes.
On the other hand, fixing by way of press fitting requires compliance with the strict bore size tolerance requirements as well as reduction of the roughness of the bore surface of an inner rotor in order to ensure a proper press-fitting margin during the cutting process of the bore surface of the inner rotor into which a shaft is press-fitted. For this reason, there has been a problem that the processing cost of such inner rotor increases. Particularly, when processing an elongated inner rotor, a bore surface cutting distance in an axial direction increases, resulting in a higher processing cost.